The present invention relates to non-invasive methods for determining the blood pressure of a subject. More particularly, the invention relates to an improved method and apparatus for making oscillometric measurements of systolic blood pressure.
Physicians and others monitor various physiological parameters in their patients and in other subjects. Such monitoring is an important tool in evaluating patients"" health. The monitoring of cardiovascular function is particularly valuable and is performed on a very widespread basis. Accurate measurement of blood pressure (xe2x80x9cBPxe2x80x9d) and other physiological parameters allows for more precise diagnosis of medical problems. For example, accurate measurement of BP is important in the correct diagnosis of hypertension.
There are various ways to measure BP. For example, BP may be measured directly in the aorta or in other arterial blood vessels. This may be done, for example, by inserting into an arterial blood vessel a probe, such as a needle or catheter which bears, or is attached to, a pressure transducer. The transducer measures the actual pressure of the blood within the blood vessel. Although it is ideal to have directly-measured BP values for diagnostic purposes, procedures for directly measuring BP are invasive and are normally restricted to critical care environments such as operating rooms.
A variety of indirect or non-invasive techniques for measuring BP have been developed and include tonometric, auscultatory, and oscillometric methods. The tonometric method typically involves a transducer which includes an array of pressure sensitive elements positioned over a superficial artery. xe2x80x9cHold downxe2x80x9d forces are applied to the transducer so as to flatten the wall of the underlying artery without occluding the artery. The pressure measured by the pressure sensitive elements is dependent upon the hold down pressure used to press the transducer against the skin of the patient.
Tonometric systems measure a reference pressure directly from a superficial artery such as the radial artery at the wrist and correlate this reference pressure with the arterial pressure. However, because the ratio of pressure outside the artery to the pressure inside the artery, known as xe2x80x9cgainxe2x80x9d, must be known and constant, tonometric systems are not reliably accurate. Furthermore, if a patient moves, recalibration of the tonometric system is required because the system may experience a change in gain. Because the accuracy of tonometric systems depends upon the accurate positioning of a transducer over the underlying artery, placement of the transducer is critical. Furthermore, achieving proper placement of the transducer in tonometric systems is time-consuming and prone to error.
The auscultatory method involves inflating a cuff placed around a limb of the patient. Following inflation of the cuff, the cuff is permitted to deflate. Systolic blood pressure (xe2x80x9cSBPxe2x80x9d) is taken to be the cuff pressure at which Korotkoff sounds begin to occur as the cuff is deflated. Diastolic blood pressure (xe2x80x9cDBPxe2x80x9d) is taken to be the cuff pressure at which the Korotkoff sounds become muffled or disappear. The auscultatory method requires a judgment to be made as to when the Korotkoff sounds start and when they stop. This judgment is made when the Korotkoff sounds are at their very lowest. As a result, the auscultatory method is subject to inaccuracies due to low signal-to-noise ratio. Another recognized major disadvantage of the auscultatory method is that its accuracy degrades severely with hypotension and obesity. It is also unreliable in infants and children.
The oscillometric method also involves the inflation of a cuff placed around a limb of the patient. In this method, the cuff is deflated slowly and the pressure within the cuff is continuously monitored. The oscillometric method measures and records the amplitude of pressure oscillations in the cuff caused by blood pressure pulses in underlying arteries. As the cuff is deflated, the pressure within the cuff exhibits a certain pressure versus time waveform (FIG. 1A). The waveform can be separated into two components, a decaying component (the applied cuff pressure, Cxe2x80x94FIG. 1C) and an oscillating component (the pressure pulse amplitudes, Axe2x80x94FIG. 1B).
The oscillating component may be represented by a curve known by those in the art as the xe2x80x9coscillometric envelopexe2x80x9d as shown in dotted line in FIG. 1B. The oscillometric envelope starts at a low value when the cuff is inflated to a level beyond the patient""s SBP and then increases to a peak value (Amax) as the cuff pressure is reduced. Once the envelope has reached Amax, the envelope then decays as the cuff pressure continues to decrease. At Amax the mean pressure in the cuff is roughly equal to the patient""s mean arterial blood pressure (xe2x80x9cMAPxe2x80x9d).
SBP, MAP and DBP values can be determined from the data obtained by monitoring the pressure within the cuff while the cuff is slowly deflated. Again, the mean arterial blood pressure value, MAP, can be estimated as the applied cuff pressure at the point in time when the peak, Amax, of the oscillometric envelope occurs (FIG. 1C). SBP may be determined as the cuff pressure corresponding to the amplitude on the systolic side (before peak amplitude Amax) of the oscillometric envelope which is equal to a certain percentage of the peak amplitude Amax. This percentage is known by those skilled in the art as the systolic Parameter Identification Point (xe2x80x9cPIPxe2x80x9d), and is generally considered to be about 55%. Similarly, DBP may be determined as the cuff pressure corresponding to the amplitude on the diastolic side (after peak amplitude Amax) of the oscillometric envelope which is equal to a certain percentage of the peak amplitude Amax. This percentage is known as the diastolic PIP, which is generally considered to be close to 72%.
So, the oscillometric method uses fixed PIP""s to calculate SBP and DBP values from Amax. Automated BP monitors using the oscillometric technique use these fixed PIP""s in their algorithms to calculate these BP figures. It has been known for some time, however, that the oscillometric method has the disadvantage, using these fixed PIP""s, of inaccuracy under the most important of circumstances, i.e., when measuring blood pressure of hypertensive patients. Specifically, using fixed PIP""s, and especially a fixed systolic PIP, tends to cause most BP measuring devices to underestimate BP at higher pressures.
Baker et al. address the problem of using fixed PIP ratios in U.S. Pat. No. 5,339,819, xe2x80x9cMethod for Determining Blood Pressure Utilizing a Neural Networkxe2x80x9d, and also in WO 92/03966, xe2x80x9cMethod and apparatus for determining blood pressurexe2x80x9d. Their solution to the problem is to train a neural network to recognize or map the relationship between sets of oscillometric envelope input data and the desired directly-measured blood pressure. The neural network is trained to analyse many data points on a single oscillometric envelope and thus has the advantage of not being entirely dependent upon a small number of parameters such as MAP and PIPs. However, neural networks have a disadvantage of being complex to design and train, and also costly to implement. A further disadvantage is that neural network performance is generally limited by the amount of training and the type of training data; neural networks may not perform well with new input data which they have not seen before.
Accordingly, an improved, more accurate method of blood pressure measurement which has the advantages of the oscillometric technique, but which does not underestimate higher pressures, is desirable.
The present invention provides an improved method and apparatus for measuring blood pressure in a subject, and in particular, systolic blood pressure.
A method according to a basic embodiment of the invention comprises the steps of: obtaining an estimate of mean arterial pressure (xe2x80x9cMAPxe2x80x9d) by measuring the cuff pressure at the peak of the oscillometric envelope using an oscillometric technique, and optionally also obtaining an estimate of systolic blood pressure (xe2x80x9cSBPxe2x80x9d) using a standard fixed-PIP oscillometric technique; determining, in a departure from the standard oscillometric technique, a new systolic parameter identification point (xe2x80x9cPIPSBPxe2x80x9d) and/or a new diastolic parameter identification point (xe2x80x9cPIPDBPxe2x80x9d), these new PIP""s being not fixed constants, but rather, varying functions of estimated MAP and/or estimated SBP; and then determining SBP and DBP by using an oscillometric technique, but using these new variable PIPSBP and PIPDBP in place of the usual fixed PIP""s.
In a more detailed embodiment of the method, blood pressure is measured by placing a blood pressure cuff around the limb of a subject, inflating it to occlude the flow of blood in that limb, and then slowly deflating the cuff while continuously collecting instantaneous cuff pressure data; extracting from the cuff pressure data the Pulse Amplitudes which are the oscillating component of the cuff pressure and are due to the blood pressure pulses, and the average Cuff Pressure, which is the decaying component of the cuff pressure, and is due to the pressure applied to said blood pressure cuff, and representing this Cuff Pressure and Pulse Amplitude data in a pressure versus time waveform, with the sequence of discrete Pulse Amplitudes being represented by an oscillometric envelope which may be interpolated or smoothed or both for high resolution or noise reduction or both.
A value for the peak amplitude Amax of the oscillometric envelope is determined, and the cuff pressure which corresponds in time with Amax is also determined, this pressure representing the estimated mean arterial pressure MAP of the subject.
PIPSBP and PIPDBP values are then calculated as functions of the MAP value; the calculation ASBP=Amax*PIPSBP is performed to determine a systolic amplitude value ASBP; SBP is determined to be the cuff pressure C which corresponds in time to ASBP on the systolic side of the oscillometric envelope; the calculation ADBP=Amax*PIPDBP is performed to determine a diastolic amplitude value ADBP; and DBP is determined to be the cuff pressure C which corresponds in time to ADBP on the diastolic side of the oscillometric envelope.
In a preferred embodiment, PIPSBP is calculated in accordance with the following piecewise linear function:
if (MAPxe2x89xa6A mmHg), then PIPSBP=xcex1;
else if (MAPxe2x89xa7B mmHg), then PIPSBP=xcex2;       else    ⁢          xe2x80x83        ⁢          PIP      SBP        =      α    -          (                                    α            -            β                                B            -            A                          xc3x97                  (                      MAP            -            A                    )                    )      
where A is a pressure in the range of 90 to 110 mmHg and is preferably 100 mmHg;
xcex1 is a number in the range of 0.5 to 0.66 and is preferably 0.58;
B is a pressure in the range of 130 to 150 mmHg and is preferably 140 mmHg; and
xcex2 is a number in the range of 0.30 to 0.46 and is preferably 0.38.
In another embodiment, PIPSBP is calculated in accordance with the following exponential function:       PIP    SBP    =      A    -          B              1        +                  Ce                      (                          D              xc3x97              MAP                        )                              
A, B, C, and D being numeric constants where:
A is in the range of 0.50 to 0.66, and is preferably 0.58;
B is in the range of 0.04 to 0.36, and is preferably 0.2;
C is in the range of 400 to 4.3xc3x971015, and is preferably 540,000; and
D is in the range of xe2x88x920.30 to xe2x88x920.05, and is preferably xe2x88x920.11.
In this embodiment, C and D may be related by the equation   C  =      (          1              e                  (                      D            xc3x97            E                    )                      )  
where E is a constant in the range of 110 to 130.
In a third embodiment, PIPSBP is calculated in accordance with the following polynomial function:
PIPSBP=Axc3x97MAP3+Bxc3x97MAP2+Cxc3x97MAP+D
A, B, C, and D being numeric constants, where:
A is in the range of 5.90xc3x9710xe2x88x927 to 6.10xc3x9710xe2x88x927;
B is in the range of xe2x88x922.2xc3x9710xe2x88x924 to xe2x88x922.02xc3x9710xe2x88x924;
C is in the range of 1.84xc3x9710xe2x88x922 to 2.35xc3x9710xe2x88x922; and
D is in the range of xe2x88x929.00xc3x9710xe2x88x922 to 3.5xc3x9710xe2x88x923.
In this embodiment, A is most preferably 6.00xc3x9710xe2x88x927; B is most preferably xe2x88x922.09xc3x9710xe2x88x924; C is most preferably 2.06xc3x9710xe2x88x922; and D is most preferably xe2x88x923.22xc3x9710xe2x88x922.
The invention also provides an apparatus for implementing the new method, the apparatus having a microprocessor, a program memory accessible by the microprocessor, a first software program component stored within the program memory for operating the apparatus, and a data memory connected to the microprocessor for storing data from the microprocessor. A blood pressure measurement subsystem is also provided which acts under the control of the first software program component. This subsystem periodically acquires, and provides to the microprocessor, instantaneous pressure values representing the pressure within a blood pressure cuff placed on a limb of a subject.
A second software program component stored within the program memory extracts, from the instantaneous pressure values, data relating to cuff pressure and a pulse amplitude with respect to time. This data is stored by the microprocessor into the data memory. A third software program component is also provided for determining MAP from the cuff pressure and pulse amplitude data. A fourth software program component is provided for determining PIPSBP and PIPDPB as functions of the MAP. A fifth software program component is provided for determining SBP and DBP values from the pressure and amplitude data stored in the data memory, using said PIPSBP and PIPDBP.
In a preferred embodiment of the system, the program memory is a ROM and the data memory may be any suitable storage means such as a RAM, EEPROM, or a disc drive. The first, second, third, fourth, and fifth software program components may conveniently be contained within a single software program.